Fabry disease is an X-linked disorder of glycosphingolipid metabolism caused by a deficiency of .alpha.-galactosidase A (.alpha.-Gal A). Human .alpha.-Gal A is a lysosomal enzyme that catalyzes the hydrolysis of .alpha.-galactosidic linkages of glycoconjugates. An X-linked inborn deficiency of this hydrolase, which is the cause of Fabry disease, leads to the accumulation of neutral glycosphingolipids, predominantly ceramide trihexoxide, primarily in the plasma and in the lysosomes of the vascular endothelium. Affected hemizygous males show various clinical symptoms, such as acroparesthesias, angiokeratoma, hypohidrosis, corneal and lenticular opacities, and progressive vascular disease of the kidney, heart, and brain, leading to death in early adulthood.
Several lines of evidence suggest that enzyme replacement therapy may be beneficial for patients with Fabry disease. For example, it has been demonstrated in cell cultures of fibroblasts obtained from patients with this disease that .alpha.-Gal A present in the culture medium is specifically transported to lysosomes. Moreover, the enzyme has been administered to patients with Fabry disease using infusions of normal plasma (Mapes et al., Science, 169, 987 (1970)); .alpha.-Gal A purified from placenta (Brady et al., New Eng. J. Med., 279, 1163 (1973)); and .alpha.-Gal A purified from spleen or plasma (Desnick et al., Processor. Natl. Acad. Sci. USA, 76, 5326 (1979)). In one study (Desnick et al., supra), intravenous injection of purified enzyme resulted in a transient reduction in the plasma levels of the substrate globtriaosylceramide. However, insufficient quantities of the purified human enzyme were available for further study.
The structure of the .alpha.-Gal A gene has been determined. The 14 kb .alpha.-Gal A genomic sequence contains 7 exons encoding a 429 amino acid polypeptide, including an NH.sub.2 -terminal 31-residue signal peptide. Studies on unrelated families with Fabry disease have shown a variety of molecular changes in the .alpha.-Gal A gene in affected individuals. More than 70 mutations in the coding region of the .alpha.-Gal A gene have been reported, the majority of which result in a classical phenotype with no .alpha.-Gal A activity. However, a few mutations that result in single amino acid substitutions in the carboxy (C)-terminal region of .alpha.-Gal A lead to an atypical variant of Fabry disease in males, with manifestations limited to the heart (H. Sakuraba et al., Am. J. Hum. Genet., 47, 784 (1990); W. V. Scheidt et al., New Engl. J. of Med., 324, 394 (1991); Y. Nagao et al., Clin. Genet., 39, 233 (1991); S. Nakao et al., New Engl. J of Med., 333, 288 (1995)). It has been suggested that this variant type might be more common than previously believed, occurring in male patients with unexplained left ventricular hypertrophy (S. Nakao et al., New Engl. J. of Med., 333, 288 (1995)).
Several mutations of the .alpha.-Gal A gene that result in the introduction of premature stop codons have also been described, including a mutant .alpha.-Gal A gene termed E398X which encodes a polypeptide that lacks 32 amino acid residues of the C-terninus of .alpha.-Gal A. All hemizygous patients with these mutations are reported to manifest a classical phenotype (C. Meaney et al., Hum. Mol. Genet., 3, 1019 (1994); M. E. Christine et al., Hum. Mol. Genet., 3, 1795 (1994)). While some investigators have speculated that 26 or 28 amino acid residues from the C-terminus of .alpha.-Gal A might be proteolytically cleaved to generate the final polypeptide product present in cells (Quinn et al., Gene, 58, 177 (1987) and Bishop et al., Proc. Natl. Acad. Sci. USA, 85, 3903 (1988)), it remains uncertain whether the C-terminus of .alpha.-Gal A is necessary for enzymatic activity.
Thus, a continuing need exists for agents which have increased enzymatic activity relative to wild type .alpha.-Gal A.